1. Field of the Invention
The present invention relates to an optical film. In particular, the present invention relates to a diffusion film applicable to a backlight module.
2. Description of the Prior Art
Since a liquid crystal panel cannot emit light by itself, a backlight module acting as a light source is an important element for the displaying function of a LCD and is extremely important for enhancing the luminance of the LCD. Currently, it has become the most economic and convenient solution to employ various optical films in the backlight module to enhance the brightness of LCD panels to optimize the use efficiency of the light source to be applied most efficiently, without altering any element design or consuming additional energy. FIG. 1 is a simplified schematic illustration of the backlight module comprising various optical films. As shown in FIG. 1, the optical films contained in a typical backlight module include a reflective film (1) positioned on the lower side of a light guide (2); and other optical films positioned on the upper side of the light guide (2), which include from the bottom to the top a lower diffusion film (3), brightness enhancing films (4) and (5), and a protective diffusion film (6) in sequence.
The main function of the brightness enhancing film involves gathering the scattered light rays emitted from the light guide in all directions by refraction and internal total reflection and converging the light rays in the on-axis direction of about ±35 degrees to enhance the luminance of the LCD. The brightness enhancing film achieves the effect of gathering light by means of regular or irregular prismatic microstructures. However, if the prismatic structures are contacted with the panel, scratching tends to occur, thus affecting the optical properties. Presently, there are no other solutions in the industry but using a protective diffusion film (which is also referred to as an “upper diffusion film”) to prevent the impairment between the brightness enhancing film and the panel caused by the vibration during transportation.
Moreover, the functions of the protective diffusion film also include scattering light passing through the brightness enhancing film to obtain a uniform luminance, eliminating the pattern of the brightness enhancing film and protecting the brightness enhancing film. The operating principle of the protective diffusion film is to make light pass through two media with different refractive indexes, and thus the light will be refracted, reflected and scattered to achieve the effect of optics diffusion. Conventionally, the protective diffusion film includes two types, i.e., the inner-diffusion type and the surface-diffusion type. The inner-diffusion type is consisting of polycarbonate (PC), polystyrene or polymethyl methacrylate (PMMA) resins with organic or inorganic particles. By the utilization of diffusion particles, the light will be refracted and scattered because it passed through two media with different refractive indexes, so that the linear light emitted from the light source can be diffused into a uniform surface light. That is, the light from a below light source is scattered by the diffusion film, uniformly dispersed upward, and then emitted. The surface-diffusion type reflects and refracts light by roughening the surface of a transparent resin plate. However, the surface-diffusion type is time-consuming and complicated in preparation procedure and is relative expensive in cost.
It is known in the art that the inner-diffusion type of protective diffusion films usually utilize diffusion particles with a wide particle size distribution and different particle sizes. That is, the particle size distribution of the diffusion particles (y represents the quantity of particles, while x represents the particle size) is a wide single-peak distribution. For example, when the particles used have a mean particle size of about 15 μm, the particle size distribution of the particles generally ranges from about 1 μm to about 30 μm. It is also known in the art that a mixture of more than two groups of particles with different mean particle sizes can be used as diffusion particles. That is, the particle size distribution of the diffusion particles (y represents the quantity of particles, while x represents the particle size) is a two (multi)-peak distribution. Although the light diffusion effect can be improved by using diffusion particles having a wide single-peak particle size distribution or a two (multi)-peak particle size distribution, the light will be scattered randomly due to the different particle sizes of the particles, and as a result, the light source cannot be efficiently utilized.
It is known that, if the diffusion particles in the diffusion film are aggregated or adhered to each other, not only is the light diffusion uniformity affected, but dark spots are also likely to be generated on the surface of the display. In order to solve the above problems, U.S. Pat. No. 7,218,450 B2 discloses using one or more organic or inorganic particles with a single distribution as diffusion particles with certain parameters, including the lamination ratio, particle size of the flocculated particles, and when two kinds of particles with a single distribution are used, the mean particle sizes of the two kinds of particles with a single distribution, that meet special formulae. 95% of the particles with a single distribution used in U.S. Pat. No. 7,218,450 B2 have a particle size ranging within ±15% of the mean particle size. U.S. Pat. No. 7,218,450 B2 further teaches using diffusion particles having a narrow particle size distribution, but is silent on the crosslinking degree of the diffusion particles. In fact, an insufficient crosslinking degree of the diffusion particles will inevitably cause some problems. For example, particles with a low crosslinking degree are likely to interact with the solvent in the binder and thus swell up. Therefore, compared with the particles with a high crosslinking degree, the particles with a low crosslinking degree will have a low solvent resistance. Furthermore, as the volume of the particles with a low crosslinking degree is changed because the solvent is absorbed, the optical properties of the particles become unstable, and the viscosity on the surfaces of the particles increases, so that the particles are easily aggregated with each other, thereby further affecting the coating processability and light diffusion effects of the diffusion film.
Therefore, in current technology, although the diffusion efficiency of protective diffusion films meet current demands, their luminance is not sufficient for the application in LCDs. Consequently, for current technology, it is necessary to further enhance the luminance of protective diffusion films, thereby increasing the whole brightness of LCDs.